Not Applicable.
1. Field of the Invention
The present invention relates generally to methods and apparatus for drilling an offshore well from a platform, and more particularly, to methods and apparatus for the open hole drilling of a subsea borehole using lightweight pipe components, and still more particularly, to methods and apparatus for drilling a conductor casing borehole through a riser with a lightweight drill string.
2. Description of the Related Art
Offshore hydrocarbon drilling and producing operations are typically conducted from a drilling rig located either on a bottom-founded offshore platform or on a floating platform. A bottom-founded platform extends from the seafloor upwardly to a deck located above the surface of the water, and at least a portion of the weight of the platform is supported by the seafloor. In contrast, a floating platform is a ship, vessel, or other structure, such as a tension-leg platform, for example, in which the weight of the platform is supported by water buoyancy.
In recent years, exploration and production of offshore crude oil and natural gas reservoirs has expanded into ever-deeper waters. Successful drilling operations have been conducted in deep waters of at least 3,000 feet deep, and ultra-deep waters ranging from 5,500 to 10,000 feet deep. With increasing water depths, drilling operations conducted from moored or dynamically positioned floating platforms have become more prevalent since economic and engineering considerations militate against the use of bottom-founded drilling platforms commonly used in shallow water.
Regardless of whether a bottom-founded or floating platform is used, conventional methods for drilling an offshore well are similar. In such operations, the platform supports a drilling rig and associated equipment, and must include adequate deck space for pipe storage and handling. The platform is positioned near the wellsite, and a drill string, typically formed of jointed steel pipe that is threaded together one joint at a time, conveys a bottom-hole drilling assembly (BHA) from the platform to the seafloor. A drill bit, disposed at the terminal end of the BHA, drills the well.
When drilling from a floating platform, the upper portion of the well is drilled by open hole drilling in that no conduit is provided for the returns to flow to the platform. Therefore, in open hole drilling the returns, i.e. the drilling fluid, cuttings, and well fluids, are discharged onto the seafloor and are not conveyed to the surface. To drill the initial upper portion of the well, the drill string typically extends unsupported through the water to the seafloor without a riser. In more detail, first an outer casing, known as xe2x80x9cstructural casingxe2x80x9d, typically having a diameter of 30-inches to 36-inches, is installed in the uppermost section of the well, with a low-pressure wellhead housing connected thereto. In soft formations, the structural casing is typically jetted into place. In this process, an assembly is lowered to the seafloor on a conventional drill string. The assembly includes the structural casing, and typically, a BHA with drill collars, a downhole motor, and a drill bit. The bit is positioned just below the bottom end of the structural casing and is sized to drill a borehole with a slightly smaller diameter than the diameter of the casing. As the borehole is drilled, the structural casing moves downwardly with the BHA. The weight of the structural casing and BHA drives the casing into the sediments. The structural casing, in its final position, generally extends downwardly to a depth of 150 to 400 feet, depending upon the formation conditions and the final well design. After the structural casing is in place, it is released from the drill string and BHA. The drill string and BHA are then tripped back to the platform, or are, in some cases, lowered to drill below the structural casing.
In more competent formations, the structural casing is similar, but it is installed in a two-step process. First, a borehole larger than the structural casing is drilled. Then the structural casing is run into the borehole and cemented into place. Typically, the low-pressure wellhead housing is connected to the upper end of the structural casing and installed at the same time, such that the structural casing extends below the seafloor with the low-pressure wellhead housing above the seafloor.
Once the structural casing and the low-pressure wellhead housing are installed, the BHA on the drill string drills downwardly below the structural casing to drill a new borehole section using open hole drilling for an intermediate casing, known as xe2x80x9cconductor casing,xe2x80x9d which is typically 20-inches in diameter. Thus, the structural casing guides the BHA as it begins to drill the conductor casing interval. During open hole drilling, returns of the drilling fluid and cuttings are discharged onto the seafloor.
After the borehole section for the conductor casing is drilled, the BHA is tripped to the surface. Then conductor casing, with a high-pressure wellhead housing connected to its upper end, and a float valve disposed in its lower end, is run into the drilled conductor borehole section extending below the structural casing. The conductor casing is cemented into place in a well known manner, with the float valve preventing cement from flowing upwardly into the conductor casing after cement placement. The conductor casing generally extends downwardly to a depth of 1,000 to 3,000 feet below the seafloor, depending on the formation conditions and the final well design. The high-pressure wellhead housing engages the low-pressure wellhead housing to form the subsea wellhead, thereby completing the riserless portion of the drilling operations. A subsea blowout preventer (BOP) stack is typically conveyed down to the seafloor by a riser and latched onto the subsea wellhead housing. The riser is thereby installed with its lower end connected to the subsea wellhead via the BOP stack and the riser extending to the platform at the surface. Subsequent casing strings are hung and well operations are conducted through the subsea wellhead.
Riserless drilling, as described above for drilling the conductor casing borehole, is conventionally performed using a drill string formed of steel pipe joints having a size and weight sufficient to withstand the lateral forces imposed by water currents. However, this conventional method of riserless drilling has a number of disadvantages, especially when drilling from a floating platform in deep or ultra-deep waters.
Once the well reaches a certain depth, further drilling requires the use of a weighted drilling fluid to maintain control of downhole pressures, and such drilling fluids are costly enough to warrant returning the drilling fluid to the platform for cleaning so that the same drilling fluid may be recirculated for further drilling. Thus, after the riserless drilling portion of the well has been drilled and cased, a low-pressure riser, formed by joining sections of casing or pipe that is typically 21-inches in diameter, is deployed between the floating platform and the wellhead equipment. The riser is provided to guide the drill string to the wellhead equipment for conducting further well drilling operations, and to provide a conduit for returning drilling fluid from the well to the floating platform.
Once the riser is in place, the drill string and BHA are lowered through the riser, the subsea wellhead, and the conductor casing to drill through the float valve into the seafloor to form another borehole section for another string of casing. The next casing, known as xe2x80x9csurface casing,xe2x80x9d which is typically 13xe2x85x9c to 16 inches in diameter, is lowered into the drilled borehole and cemented into place via conventional procedures. The surface casing generally extends to a depth of 2,500 to 5,000 feet below the seafloor, depending on the formation characteristics and final well design. Subsequent, smaller diameter, intermediate casing strings may be installed below the surface casing.
This conventional method of drilling with a riser from a platform has a number of disadvantages, especially when drilling from a floating platform in deep or ultra-deep waters. First, the required size and capacity of the platform is largely based on the depth of water, and the corresponding amount of pipe required to drill the well. The larger the pipe, and the more pipe required to form the riser, the greater the weight and space requirements of the drilling rig and floating platform. To handle the weight of a large and long drill string, and a large and long riser, the floating platform must be equipped with a conventional drilling rig and must have significant deck space for storing and handling the large amount of pipe for the drilling operation.
Thus, as water depth increases, larger floating platforms are required for larger drilling rigs to handle and support the added weight of the pipe due to the greater depth and to store the additional pipe, thereby significantly increasing the costs of drilling as water depth increases. Further, tripping into and out of the well with jointed pipe is very time-consuming since each joint of pipe must be threaded and/or unthreaded to the pipe string extending through the water and into the well. As an additional concern, the number of trips into and out of the well, and the heave and roll of the floating platform, impose fatigue stresses on the metal pipe extending down to the seafloor from the floating platform. Heave compensators on the floating platforms compensate for the heave of the floating platform and help to protect the pipe from excess fatigue.
Various improvements may be made to overcome the deficiencies of conventional drilling operations. It would be advantageous to reduce the size of the platform, particularly floating platforms required for deep water. One way to enable the use of a smaller platform would be to reduce the capacity requirement of the hoisting system, which would allow reduction of the drilling rig size, or would allow replacement of the drilling rig with a smaller capacity hoisting system. Further, the diameter and therefore the weight of the pipe, such as drill pipe, casing, and risers, could be reduced, thereby no longer requiring a large drilling rig to handle the pipe, and no longer requiring large storage space on the platform for the pipe. To achieve these objectives, it would be preferred to eliminate large risers and to use smaller risers. This will reduce the required drilling rig size and the amount of storage space required. When the riser diameter is reduced to the preferred smaller diameter, a conventionally sized drill string is too large to extend through the riser. For this reason, a smaller diameter drill string must be used when drilling through the preferred smaller diameter riser. A reduction in drill string diameter typically results in a proportional reduction in the weight of the drill string. Thus, in order to maximize efficiency, it would be preferable to use the same, smaller diameter, lighter drill string for conducting the riserless drilling operations described above. In addition to enabling the use of a smaller riser, the use of a smaller, lighter drill string is preferable because its lighter weight directly reduces the vessel size requirement.
For these reasons, it would be preferable to use a lighter weight drill string. It would be more preferable to use a non-jointed, continuous lighter weight drill string such as coiled tubing stored on a reel, thereby reducing the deck space required to store the drill string. Further, because a coiled tubing drill string is a continuous, single length of tubing that may be continuously fed from the reel into the water and down into the well, the time required to connect and disconnect the joints of a conventional drill string is eliminated, thereby significantly reducing the overall time required to conduct drilling operations. It would be still more preferable to use a non-metal coiled tubing drill string, such as the composite coiled tubing disclosed in U.S. Pat. No. 6,296,066 to Terry et al., hereby incorporated herein by reference for all purposes. Composite coiled tubing is preferable to metal pipe or metal coiled tubing because it weighs less and is substantially less subject to fatigue inducing stress variations due to trips into and out of the well and movement of the floating platform.
Drill string weight may be reduced by reducing the wall thickness of the drill string, or by altering the material that forms the drill string, such as by using a lightweight metal like titanium, or by using a lightweight composite material. A composite coiled tubing drill string may be formed of helically wounded or braided fiber reinforced thermoplastic or fiber reinforced thermosetting polymer or epoxy, for example. It should be appreciated that one or more of these concepts may be combined to reduce drill string weight, resulting in a lightweight drill string. However, as the drill string is made lighter, it becomes more susceptible to the effects of water currents. The lighter the drill string, the more severe the effects. Because water currents vary with depth and with time, and because the variability of the currents increases with increasing water depth, it is difficult to precisely predict deepwater currents and thus to design for their adverse effects. In particular, water currents have various impacts on a lightweight drill string and BHA during riserless drilling. As used herein, a lightweight drill string is defined as a drill string, which is lighter than that used in conventional drilling, and which requires alternative systems and methods to conduct riserless drilling due to factors associated with its light weight, such as its response to water currents.
Conventional riserless drilling system and methods cannot be used with a lightweight drill string due to the conventional systems"" inability to counteract the effects of the water currents on the lightweight drill string. Because the drill string is laterally constrained at the platform and at the point of entry into the borehole at the seafloor the drill string will bow as the water currents impose lateral forces against it. As water depth increases, the bowing effect of the drill string increases because there is a greater length of the drill string upon which the water currents act. The bowing of the drill string exerts an upward force on the BHA, tending to pull the BHA out of the borehole. This upward force reduces weight-on-bit (WOB) and possibly lifts the bit off bottom, thereby preventing successful drilling.
Furthermore, as the weight of the drill string is reduced and the water depth increases, the tendency of the drill string to kink increases, particularly at the floating platform and at the seafloor where the drill string is laterally constrained. Thus, if the drill string bends too sharply, it will kink, and ultimately fail. Therefore, it would be advantageous to provide methods and apparatus to counteract the effects of water currents such that successful drilling of the conductor casing borehole can be achieved using a lightweight drill string.
Another disadvantage of the conventional method described above is that two different pipe trips are performed to drill a borehole for casing and to install the casing, respectively, such that the open borehole could experience catastrophic failure due to shallow water flows, making it impossible for the casing to be run into the borehole. The longer the delay between drilling the borehole and running the casing into the borehole, the more likely the borehole will collapse before the casing can be run in. In ultra deep water of 10,000 feet, for example, a single roundtrip of the drill string can take up to an entire day. Thus, it would be advantageous to minimize the delay between drilling a borehole and running casing into that borehole.
The present invention overcomes the deficiencies of the prior art.
The methods and apparatus of the preferred embodiments are for the open hole drilling of a borehole from an offshore platform and through a cased borehole at the seafloor. The drilling assembly includes a guide assembly extending from the platform to the seafloor and having a lower end extending into the cased borehole; and a bottomhole assembly disposed on a lightweight drill string extending through the guide assembly for drilling the borehole. The guide assembly is a pipe string and preferably includes a casing on the lower end and a riser attached to the upper end of the casing. The lightweight drill string may be a lightweight jointed pipe, or a metal coiled tubing, or preferably a composite coiled tubing. The bottomhole assembly includes a formation displacement member adapted to drill a borehole diameter greater than the diameter of the casing on the lower end of the guide assembly. For example, the formation displacement member may include a bi-center bit, or a conventional bit with an underreamer, or a conventional bit with a winged reamer. A drilling fluid is used that flows through the drill string and the bottomhole assembly, through a fluid passageway around the bottomhole assembly, and between the lower end and the cased borehole into the sea.
The present invention further comprises a method of open hole drilling of a new borehole through a cased borehole at the seafloor, the method comprising lowering a guide assembly from a platform through a depth of water; stabbing the guide assembly into the cased borehole; extending a bottomhole assembly suspended on a drill string through the guide assembly; drilling the new borehole; and lowering the guide assembly into the new borehole. In one embodiment, the method further comprises disposing a float valve at the lower end of the guide assembly; cementing the guide assembly into the new borehole; extending the bottomhole assembly suspended on the drill string through the guide assembly; and drilling a subsea borehole below the guide assembly.